U.S. patent application number 09/872440 was filed with the patent office on 2002-12-05 for system and method for actively aligning mirrors in an optical switch.
Invention is credited to Hoke, Charles D., Lemoff, Brian E., Schroeder, Dale W..
Application Number | 20020181848 09/872440 |
Document ID | / |
Family ID | 25359578 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020181848 |
Kind Code |
A1 |
Lemoff, Brian E. ; et
al. |
December 5, 2002 |
System and method for actively aligning mirrors in an optical
switch
Abstract
In a 3D MEMS optical switch titling mirrors are actively aligned
to minimize losses in optical power. In one embodiment the optical
signals in the output fibers are tapped and detected and the sensed
outputs are used by a control circuit with a feedback loop that
adjusts the alignment signals sent to the MEMS actuators. In a
second embodiment, an emitter which is either a single LED or laser
diode is optically coupled to the output fibers for injecting
alignment beams back into the fibers. The alignment beams have a
frequency bandwidth outside that of the information beams. The
alignment beams are detected at the input fibers via directional
optical couplers and the sensed outputs are used by a control
circuit with a feedback loop to adjust the alignment signals. The
alignment signals are dithered and their phase and amplitude shifts
are used to generate the appropriate feedback signals.
Inventors: |
Lemoff, Brian E.; (Union
City, CA) ; Hoke, Charles D.; (Menlo Park, CA)
; Schroeder, Dale W.; (Scotts Valley, CA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC.
INTELLECTUAL PROPERTY ADMINISTRATION, LEGAL DEPT.
P.O. BOX 7599
M/S DL429
LOVELAND
CO
80537-0599
US
|
Family ID: |
25359578 |
Appl. No.: |
09/872440 |
Filed: |
June 1, 2001 |
Current U.S.
Class: |
385/18 ;
385/17 |
Current CPC
Class: |
G02B 6/4225 20130101;
G02B 6/3518 20130101; G02B 6/3582 20130101; G02B 6/3556 20130101;
G02B 6/359 20130101 |
Class at
Publication: |
385/18 ;
385/17 |
International
Class: |
G02B 006/35 |
Claims
We claim:
1. A system for actively aligning mirrors in an optical switch,
comprising: a plurality of input optical fibers; a plurality of
output optical fibers; at least one array of tilting mirrors
positioned for each receiving a beam of light from a corresponding
one of the input optical fibers and directing the beam of light for
receipt by a predetermined one of the output optical fibers; a
plurality of actuators each for tilting a corresponding one of the
mirrors; a plurality of means for each redirecting a portion of the
beam of light received by a corresponding one of the output optical
fibers; a plurality of detectors each optically coupled to a
corresponding one of the portion redirecting means; and a control
circuit connected to the detectors and to the actuators that
precisely tilts each mirror to minimize losses in optical signal
power resulting from the routing of each light beam switched from a
predetermined one of the input optical fibers to a predetermined
one of the output optical fibers.
2. The system of claim 1 wherein the system includes a first array
of tilting mirrors and a second array of tilting mirrors, each
mirror in the first array receiving a beam of light from a
corresponding one of the input optical fibers and each mirror in
the second array receiving the beam of light from a corresponding
mirror in the first array and for directing the beam of light for
receipt by the predetermined one of the output optical fibers.
3. The system of claim 2 wherein the first and second arrays of
mirrors are juxtaposed opposite each other and there is a least one
lens for imaging the beam of light from a facet of the
corresponding input optical fiber onto a facet of the predetermined
output optical fiber.
4. The system of claim 1 wherein the actuators are MEMS
actuators.
5. The system of claim 1 wherein the portion redirecting means are
fractional taps.
6. The system of claim 1 wherein the fractional taps are
directional optical couplers.
7. The system of claim 1 wherein the portion redirecting means are
dichroic mirrors.
8. The system of claim 1 wherein the input optical fibers and the
output optical fibers are held in an alignment device.
9. The system of claim 1 wherein the control circuit includes a
feed-back loop.
10. The system of claim 1 wherein the actuators are selected from
the group consisting of piezo-electric transducers, electrostatic
comb devices and MEMS actuators.
11. A system for actively aligning mirrors in an optical switch,
comprising: a plurality of input optical fibers; a plurality of
output optical fibers; a first array of tilting mirrors; a second
array of titling mirrors; each mirror in the first array positioned
for receiving an information beam of light from a corresponding one
of the input optical fibers and each mirror in the second array
positioned for receiving the information beam from a corresponding
mirror in the first array and for directing the information beam
for receipt by a predetermined one of the output optical fibers; a
first plurality of actuators each for tilting a corresponding one
of the mirrors of the first array; a second plurality of actuators
each for tilting a corresponding one of the mirrors of the second
array; an emitter; a first plurality of means for each connecting
the emitter to a corresponding one of the output optical fibers for
injecting an alignment beam of light; a second plurality of means
for each connecting a corresponding one of the input optical fibers
for receiving the alignment beam transmitted therethrough; a
plurality of detectors each connected to a corresponding one of the
second plurality of connecting means for generating an electrical
signal representative of the alignment beam; and a control circuit
connected to the detectors and to the actuators that precisely
tilts each mirror to minimize losses in optical signal power
resulting from the routing of each light beam switched from a
predetermined one of the input optical fibers to a predetermined
one of the output optical fibers.
12. The system of claim 11 wherein a frequency of the alignment
beams is in a first wavelength band that is different from a second
wavelength band of the information beams.
13. The system of claim 11 wherein the first and second arrays of
mirrors are juxtaposed opposite each other and there is at least
one lens for imaging the beam of light from a facet of the
corresponding input optical fiber onto a facet of the predetermined
output optical fiber.
14. The system of claim 11 wherein the actuators are selected from
the group consisting of piezo-electric transducers, electrostatic
comb devices and MEMS actuators.
15. The system of claim 11 wherein the first and second connecting
means are directional optical couplers.
16. The system of claim 11 wherein the input optical fibers and the
output optical fibers are held in an alignment device.
17. The system of claim 11 wherein the control circuit includes a
feed-back loop that generates an alignment signal to be applied to
each of the actuators for dithering the corresponding mirror.
18. The system of claim 17 wherein a frequency of the alignment
signal is outside a frequency band of the information beam.
19. The system of claim 18 wherein the control circuit generates a
plurality of alignment signals corresponding to each channel of the
system, a channel being defined by a path of an information beam
from an input optical fiber to an output optical fiber.
20. The system of claim 17 wherein the alignment signal is
generated based on a feedback signal reflecting shifts in an
amplitude and in a phase of the alignment signal.
21. A method for actively aligning mirrors in an optical switch,
comprising the steps of: transmitting a plurality of information
light beams through free space between corresponding optical fibers
in an input bundle and an output bundle utilizing a plurality of
arrays of tilting mirrors to direct the information light beams,
each corresponding optical input fiber and optical output fiber
defining a channel; detecting a loss in optical power in each of
the channels by tapping into each optical output fiber; generating
an alignment signal for each channel based on the loss detected for
that channel; and using the alignment signal to control an actuator
associated with each tilting mirror directing the information light
beam for each channel so as to minimize the optical loss in that
channel.
22. The method of claim 21 and further comprising the step of
dithering each of the tilting mirrors with the alignment
signal.
23. The method of claim 22 wherein a frequency of the alignment
signal is outside a frequency band of the information beam.
24. The method of claim 21 wherein the alignment signal is
generated based on a feedback signal reflecting shifts in an
amplitude and in a phase of the alignment signal.
25. The method of claim 21 wherein two mirrors are each tilted in
two different directions in switching each channel and four
alignment signals are generated for each channel.
26. A method for actively aligning mirrors in an optical switch,
comprising the steps of: transmitting a plurality of information
light beams through free space between corresponding optical fibers
in an input bundle and an output bundle utilizing a plurality of
arrays of tilting mirrors to direct the information light beams,
each corresponding optical input fiber and optical output fiber
defining a channel; transmitting a plurality of alignment light
beams through free space between corresponding optical fibers in
the output bundle and the input bundle utilizing the plurality of
arrays of tilting mirrors to direct the alignment light beams;
detecting a loss in optical power of the alignment light beam in
each of the channels; generating an alignment signal for each
channel based on the loss in optical power of the light beam
detected for that channel; and using the alignment signal to
control an actuator associated with each tilting mirror directing
the information light beam for each channel so as to minimize the
optical loss in that channel.
27. The method of claim 26 and further comprising the step of
dithering each of the tilting mirrors with the alignment
signal.
28. The method of claim 27 wherein a frequency of the alignment
signal is outside a frequency band of the information beam.
29. The method of claim 26 wherein the alignment signal is
generated based on a feedback signal reflecting shifts in an
amplitude and in a phase of the alignment signal.
30. The method of claim 29 wherein two mirrors are each tilted in
two different directions in switching each channel and four
alignment signals are generated for each channel.
31. A system for actively aligning mirrors in an optical switch,
comprising: an input optical fiber; a plurality of output optical
fibers; a single tilting mirror positioned for receiving a beam of
light from the input fiber and redirecting the beam of light; an
array of tilting mirrors positioned for each receiving the beam of
light from the single tilting mirror and directing the beam of
light for receipt by a predetermined one of the output optical
fibers; a plurality of actuators each for progressively tilting a
corresponding one of the mirrors; a plurality of means for each
redirecting a portion of the beam of light received by a
corresponding one of the output optical fibers; a plurality of
detectors each optically coupled to a corresponding one of the
portion redirecting means; and a control circuit connected to the
detectors and to the actuators that precisely tilts each mirror to
minimize losses in optical signal power resulting from the routing
of each light beam switched from the input optical fiber to a
predetermined one of the output optical fibers.
32. A system for actively aligning mirrors in an optical switch,
comprising: an input optical fiber; a plurality of output optical
fibers; a single tilting mirror positioned for receiving an
information beam of light from the input fiber and redirecting the
beam of light; an array of titling mirrors; each mirror in the
array being positioned for receiving the information beam of light
from the input optical fiber and for directing the information beam
for receipt by a predetermined one of the output optical fibers; a
plurality of actuators each for tilting a corresponding one of the
mirrors; an emitter; a plurality of first means for each connecting
the emitter to a corresponding one of the output optical fibers for
injecting an alignment beam of light; second means for connecting
the input optical fiber for receiving the alignment beam
transmitted therethrough; a detector connected to the second
connecting means for generating an electrical signal representative
of the alignment beam; and a control circuit connected to the
detectors and to the actuators that precisely tilts each mirror to
minimize losses in optical signal power resulting from the routing
of each light beam switched from the input optical fiber to a
predetermined one of the output optical fibers.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to telecommunications
networks, and more particularly, to pure optical switches which
direct light pulses from one optical fiber to another without
electrical conversion.
BACKGROUND OF THE INVENTION
[0002] Telecommunications service providers continue to seek ever
greater bandwidth at ever lower prices. Their data networks must be
flexible to allow for continual upgrades, also referred to as
"provisioning". They must also designed for rapid fault recovery to
avoid service degradation and even outages. High speed optical data
networks now carry most of the long haul, and much of the
metropolitan area data traffic in developed countries. Along such
networks microprocessor controlled routers perform so-called "OEO"
transcriptions, converting optically encoded data received from
input optical fibers to electrical signals, reading destination
code, and then reconverting the electrical signals back to
optically encoded data and sending it along output optical fibers.
As transmission speeds pass 2.488 Gbits/sec (OC-48 level), this
conversion step becomes more difficult to perform and the cost of
conventional high throughput electrical switches becomes
unacceptable.
[0003] Pure optical switches direct light pulses directly from one
optical fiber to another without electrical conversion and
therefore offer the promise of eliminating much of the OEO
transcriptions in high bandwidth fiber optic data transmission
networks. Electrical routing intelligence would still be needed to
direct traffic. However, currently about eighty percent of the
traffic handled by a conventional router passes straight through
and reading the destination header in most cases is a waste of time
and system resources. By separating the control information from
the transmitted data, pure optical switching would bring
substantial increases in the throughput rate of optical data
networks.
[0004] A variety of miniature electromechanical devices have been
developed for changing the path of light in free space to direct
light pulses from one optical fiber to another optical fiber. One
promising approach utilizes three dimensional (3D)
microelectromechanical systems (HEMS). Generally speaking, MEMS
fabrication technology involves shaping a multi-layer monolithic
structure by sequentially depositing and configuring layers of a
multi-layer wafer. The wafer typically includes a plurality of
polysilicon layers that are separated by layers of silicon dioxide
and silicon nitride. The shaping of individual layers is done by
etching that is controlled by masks patterned by photolithographic
techniques. MEMS fabrication technology also entails etching
intermediate sacrificial layers of the wafer to release overlying
layers for use as thin elements that can be easily deformed and
moved. Further details of MEMS fabrication technology may be found
in a paper entitled "MEMS The Word for Optical Beam Manipulation"
published in Circuits and Devices, July 1997, pp. 11-18. See also
"Multiuser MEMS Processes (MUMPS) Introduction and Design Rules"
Rev. 4, Jul. 15, 1996 MCNC Mems Technology Applications Center,
Research Triangle Park, North Carolina 27709 by D. Keoster, R.
Majedevan, A. Shishkoff and K. Marcus.
[0005] FIG. 1 is a diagrammatic illustration of a conventional 3D
MEMS optical switch 10. A first array 12 of micro-machined mirrors
is aligned with an input optical fiber bundle 14, and juxtaposed
opposite a second array 16 of micro-machined mirrors. Electrical
command signals from a switch controller (not illustrated) cause
individual mirrors in the arrays 12 and 16 to tilt. Input light
pulses transmitted through a selected fiber in the input bundle 14
that strike an individual mirror in the first array 12 can be
directed to another specific mirror in the second array 16 and from
that mirror to a selected fiber in an output optical fiber bundle
18 aligned with the second array 16. The individual light beams
travel along Z-shaped paths 19 in free space. There is usually a
lens (not illustrated) between the first and second mirror arrays
12 and 14. The purpose of this lens is to image the facets of the
fibers in the input bundle 14 onto the facets of the fibers in the
output bundle 18. Because the light beams coming out of the fibers
in the input bundle 14 diverge, the lens is necessary to focus the
light onto the fibers in the output bundle 18. In some cases, there
are two lenses between the two arrays 12 and 14 to form a sort of
telescope in order to accomplish this imaging. The optical switch
10 has distinct advantages over electrical switches in that the
former operates completely independent of changes in the bit rate,
wavelength and polarization. 3D MEMS optical switches are targeted
for use in network cores and nodes in both long haul and
metropolitan area data networks. 2D MEMS optical switches simply
raise or lower pop-up mirrors at fixed angles to switch to a given
data port. See for example U.S. Pat. No. 5,994,159 of Aksyuk et al.
assigned to Lucent Technologies, Inc. and U.S. Pat. No. 6,097,859
of Sogarard et al. assigned to the Regents of the University of
California. In the 3D MEMS optical switch of FIG. 1, optical
signals are reflected by the first and second arrays 12 and 16 each
made of micro-machined mirrors that can each be tilted variable
amounts in two axes, bouncing an incoming optical signal from a
selected optical fiber in the input bundle 14 to a selected optical
fiber in the output bundle 18 in a manner that results in less
signal loss than in 2D MEMS optical switches.
[0006] The 3D MEMS optical switch of FIG. 1 accommodates any data
rate or transmission protocol and its architecture is more readily
scalable than 2D MEMS optical switch designs. Larger switching
capacities are achieved simply by doubling, rather than squaring,
the number of mirrors needed for the desired channel count. 2D MEMS
optical switches are really not practical beyond a 32.times.32
matrix. 3D MEMS optical switches have been commercially announced
that offer a 64.times.64 input/output capacity in a relatively
small form factor. They have high cross-talk rejection and flat
passband response and are well suited for use in
wavelength-division multiplexed (WDM) optical data networks.
[0007] While 3D MEMS optical switches show great promise, precise
angular alignment of the miniature mirrors can be difficult to
achieve. Precise alignment is needed in order to minimize optical
losses.
SUMMARY OF THE INVENTION
[0008] It is therefore the primary object of the present invention
to provide a system and method for actively aligning titling
mirrors in a pure optical switch.
[0009] In accordance with a first embodiment of our invention a
system for actively aligning mirrors in an optical switch includes
a plurality of input optical fibers, a plurality of output optical
fibers and at least one array of tilting mirrors. Each tilting
mirror receives a beam of light from a corresponding one of the
input optical fibers and directs the beam of light for receipt by a
predetermined one of the output optical fibers. A plurality of
actuators each progressively tilt a corresponding one of the
mirrors. A plurality of fractional taps such as directional optical
couplers, dichroic mirrors, optical wavelength
multiplexer/de-multiplexer devices or other devices each redirect a
portion of the beam of light received by a corresponding one of the
output optical fibers. A plurality of detectors are each optically
coupled to a corresponding one of the optical taps. A control
circuit is connected to the detectors and to the actuators and
precisely tilts each mirror to minimize losses in optical signal
power resulting from the routing of each light beam as it is
switched from a predetermined one of the input optical fibers to a
predetermined one of the output optical fibers.
[0010] In accordance with a second embodiment of our invention a
system for actively aligning mirrors in an optical switch includes
a plurality of input optical fibers, a plurality of output optical
fibers, a first array of tilting mirrors and a second array of
titling mirrors. Each mirror in the first array receives an
information beam of light from a corresponding one of the input
optical fibers and each mirror in the second array receives the
information beam from a corresponding mirror in the first array and
directs the information beam for receipt by a predetermined one of
the output optical fibers. A first plurality of actuators each
progressively tilt a corresponding one of the mirrors of the first
array. A second plurality of actuators each progressively tilt a
corresponding one of the mirrors of the second array. A first
plurality of mechanisms such as directional optical couplers each
connect an emitter to a corresponding one of the output optical
fibers for injecting an alignment beam of light. A second plurality
of mechanisms such as directional optical couplers each connect a
corresponding one of the input optical fibers for receiving the
alignment beam transmitted therethrough. A plurality of detectors
are each connected to a corresponding one of the second plurality
of directional optical couplers and each generate an electrical
signal representative of the alignment beam. A control circuit is
connected to the detectors and to the actuators. The control
circuit precisely tilts each mirror to minimize losses in optical
signal power resulting from the routing of each light beam switched
from a predetermined one of the input optical fibers to a
predetermined one of the output optical fibers.
[0011] In accordance with a first embodiment of our method for
actively aligning mirrors in an optical switch, a plurality of
information light beams are transmitted through free space between
corresponding optical fibers in an input bundle and an output
bundle utilizing a plurality of arrays of tilting mirrors to direct
the information light beams. Each corresponding optical input fiber
and optical output fiber define a channel. A loss in optical power
in each of the channels is detected by tapping into each optical
output fiber. An alignment signal is generated for each channel
based on the loss detected for that channel. The alignment signal
is utilized to control an actuator associated with each tilting
mirror to direct the information light beam for each channel so as
to minimize the optical loss in that channel.
[0012] In accordance with a second embodiment of our method for
actively aligning mirrors in an optical switch, a plurality of
information light beams are transmitted through free space between
corresponding optical fibers in an input bundle and an output
bundle utilizing a plurality of arrays of tilting mirrors to direct
the information light beams. Each corresponding optical input fiber
and optical output fiber define a channel. A plurality of alignment
light beams are transmitted through free space between
corresponding optical fibers in the output bundle and the input
bundle utilizing the plurality of arrays of tilting mirrors to
direct the alignment light beams. A loss in optical power of the
alignment light beam in each of the channels is detected. An
alignment signal is generated for each channel based on the loss in
optical power of the light beam detected for that channel. The
alignment signal is utilized to control an actuator associated with
each tilting mirror directing the information light beam for each
channel so as to minimize the optical loss in that channel.
[0013] The first and second embodiments may also be simplified to
provide 1.times.N optical switches.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a diagrammatic side elevation view illustrating a
conventional 3D MEMS optical switch.
[0015] FIG. 2 is a schematic diagram of a first embodiment of a
system for actively aligning mirrors in a 3D MEMS optical
switch.
[0016] FIG. 3 is a schematic diagram of a second embodiment of a
system for actively aligning mirrors in a 3D MEMS optical
switch.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] Referring to FIG. 2, a system 20 for actively aligning
mirrors in an optical switch includes a first bundle 22 containing
a first plurality of input optical fibers, a second bundle 24
containing a second plurality of output optical fibers, and first
and second opposing arrays of micro-machined tilting mirrors 26 and
28. The polished end faces or facets of the first bundle 22 are all
co-planar. The polished end faces or facets of the second bundle 24
are also all co-planar. The input optical fibers and the output
optical fibers of the bundles 22 and 24 are preferably held in a
conventional alignment device or coupling mechanism (not
illustrated) typically including an alignment sleeve along with
male and female parts. Each tilting mirror 26 of the first array
receives a beam of light 30 from a corresponding one of the input
optical fibers such as 22a and directs the beam of light 30 for
receipt by a predetermined one of the mirrors 28 of the second
array which directs the beam 30 to a predetermined one of the
output optical fibers such as 24a. The beam of light 30 is
illustrated as a dashed line in FIG. 2 and passes between the
bundles 22 and 24 and the mirrors 26 and 28 in so-called free
space, i.e. without passing through any physical medium other than
gas except for two lenses 31a and 31b.
[0018] The lenses 31a and 31b (FIG. 2) are positioned between the
bundles 22 and 24. The lenses 31a and 31b image the light beam 30
from the input optical fiber 22a to the output optical fiber 24a.
The lenses 31a and 31b accommodate divergence of the light beams as
they exit the input bundle 22. The lenses 31a and 31b thus image
the light from the plurality of facets of the input bundle 22 to
the facets of the output bundle 24.
[0019] A first plurality of MEMS actuators 32 (FIG. 2) each
progressively tilt a corresponding one of the mirrors 26. A second
plurality of MEMS actuators 33 each progressively tilt a
corresponding one of the mirrors 28. A plurality of fractional taps
34 each redirect a portion of the beam of light 30 received by a
corresponding one of the output optical fibers such as 24a. A
plurality of optical detectors 36 are each optically coupled to a
corresponding one of the fractional taps 34. A control circuit 38
is connected to the detectors 36 and to the MEMS actuators 32 and
33 and precisely tilts each of the mirrors 26 and 28 to minimize
losses in optical signal power resulting from the routing of each
information carrying light beam 30, as it is switched from a
predetermined one of the input optical fibers in the bundle 22 to a
predetermined one of the output optical fibers in the bundle
24.
[0020] The beneficial results of the system 20 depend on prior
proper alignment of the optical fibers in the bundles 22 and 24.
The control circuit 38 may have a feedback loop or other suitable
architecture. Directional optical couplers are one form of the
fractional taps 34. Sometimes directional couplers that have a weak
wavelength dependence are used as a fractional tap, while other
directional couplers with a strong wavelength dependence can be
used to separate different wavelengths. The embodiment of FIG. 2 is
not wavelength dependent so can utilize fractional taps 34 as a
means for each redirecting a portion of the beam of light 30
received by each one of the output optical fibers. Most of the
light in each beam 30 leaves the fractional taps 34 and is conveyed
along the fiber optic data path 39. A fraction of the light in each
beam is tapped off by the fractional taps 34 and received by the
detectors 36.
[0021] The system 20 of FIG. 2 has the disadvantage that by tapping
the optical signal being switched, loss is introduced into that
channel. Also, if no light is currently being transmitted in a
particular channel, active alignment cannot be achieved. FIG. 3 is
a schematic diagram of an alternate system 40 for actively aligning
mirrors in a 3D MEMS optical switch which overcomes the foregoing
drawbacks. The system 40 includes a first bundle 42 containing a
first plurality of input optical fibers, a second bundle 44
containing a second plurality of output optical fibers, and first
and second opposing arrays of micro-machined tilting mirrors 46 and
48. The polished end faces or facets of the first bundle 42 are all
co-planar. The polished end faces or facets of the second bundle 44
are also all co-planar. The input optical fibers and the output
optical fibers of the bundles 42 and 44 are preferably held in a
conventional alignment device or coupling mechanism (not
illustrated) typically including an alignment sleeve along with
male and female parts.
[0022] Each tilting mirror 46 (FIG. 3) of the first array receives
an information beam of light 50 from a corresponding one of the
input optical fibers such as 42a and directs the beam of light 50
for receipt by a predetermined one of the output optical fibers
such as 44a. There are actually a plurality of beams of light 50
propagating through the system 40 from left to right in FIG. 2 from
the various fibers in the input bundle 42 to the various fibers in
the output bundle 44. Only one beam of light 50 is illustrated in
FIG. 3 as a series of dashes. Each information beam of light 50 is
generated by a data source 53 and carries information typically
encoded via some form of modulation. Each beam 50 is imaged onto a
facet of a corresponding output optical fiber, such as 44a, by a
pair of lenses 51a and 51b that are positioned between the two
arrays of mirrors 46 and 48.
[0023] Referring still to FIG. 3, a first plurality of MEMS
actuators 52 each progressively tilt a corresponding one of the
mirrors 46. A second plurality of MEMS actuators 53 each
progressively tilt a corresponding one of the mirrors 48. An
emitter 54 such as a single LED or a single laser diode
simultaneously illuminates the facets of all the optical fibers of
the second bundle 44. This is accomplished using a first plurality
of directional optical couplers 56. The emitter 54 generates, via
the directional optical couplers 56, a plurality of alignment beams
of light 58 having a wavelength band different from that of the
information beams 50 carrying the data being transmitted by the
system 40. The emitter 54 is coupled to the facets of the output
optical fibers 44 via the directional optical couplers 56 to inject
the light beams 58 that are illustrated in FIG. 3 as a series of
dots and dashes. The alignment light beams 58 propagate through the
system 40 (from right to left in FIG. 3) in a direction opposite to
that of the information light beams 50. Inside the system 40, each
alignment beam 58 for each channel will propagate along the same
exact path, only in the opposite direction, as the information
light beam 50 for the same channel. The information light beams 50
continue along the fiber optic data path 59.
[0024] A second plurality of directional optical couplers 60 (FIG.
3) is used to extract the alignment beams 58 from the optical
fibers of the input bundle 42. Each of the directional optical
couplers 60 is coupled to a corresponding detector 62. The
information light beams 50 are transmitted from the data source 53
through the same directional optical couplers 60 to the optical
fibers of the input bundle 42. The output signals from the
plurality of detectors 62 are fed to a control circuit 64. The
control circuit 64 is connected to the MEMS actuators 52 and 53 and
precisely tilts each of the mirrors 46 and 48 to minimize losses in
optical signal power resulting from the routing of each information
light beam 50, as it is switched from a predetermined one of the
input optical fibers in the bundle 42 to a predetermined one of the
output optical fibers in the bundle 44. The control circuit 64 may
have a feedback loop or other suitable architecture.
[0025] The directional optical couplers 56 and 60 of the system 40
of FIG. 3 have a wavelength dependence that is selected so that the
information light beams 50 and the alignment beams 58 are correctly
routed. Of course the correct ports of each directional optical
coupler must be connected to the correct facets, emitter and
detector.
[0026] The control circuits 38 and 54 must derive a correction
signal in order to actively align the micro-machined mirrors in a
manner that minimizes loss of optical power in the switching
process. This is preferably accomplished by dithering each mirror,
via its associated MEMS actuator, with an electrical alignment
signal having a very small amplitude at a frequency outside the
frequency band of the data being transmitted. By measuring the
shift in the phase and amplitude of the alignment signal, an
appropriate feedback signal can be derived to adjust the mirror
angle. Since each switch connection for each optical channel
involves two mirrors, each having two dimensions or freedoms of
movement, four distinct dither frequencies must be utilized in the
case of a 3 MEMS optical switch.
[0027] Thus, in accordance with a first embodiment of our method,
the plurality of information light beams 30 (FIG. 2) are
transmitted through free space between corresponding optical fibers
in the input bundle 22 and the output bundle 24 utilizing the two
generally planar arrays of tilting mirrors 26 and 28 to direct the
information light beams 30. Each corresponding optical input fiber,
such as 22a, and optical output fiber, such as 24a, define a
channel. A loss in optical power in each of the channels is
detected by tapping into each optical output fiber utilizing
fractional taps 34 and optical detectors 36. An alignment signal is
generated for each channel based on the loss detected for that
channel. The alignment signal is utilized by the control circuit 38
to control MEMS actuators 32 and 33 associated the tilting mirrors
26 and 28 to direct the information light beam 30 for each channel
so as to minimize the optical loss in that channel.
[0028] In accordance with a second embodiment of our method, a
plurality of information light beams 50 are transmitted through
free space between corresponding optical fibers in the input bundle
42 and the output bundle 44 utilizing the two generally planar
arrays of tilting mirrors 56 and 58 to direct the information light
beams 50. Each corresponding optical input fiber and optical output
fiber define a channel. The plurality of alignment light beams 58
are transmitted into the output fibers in the bundle 44 via emitter
54 and directional optical couplers 56 and then through free space
to corresponding optical fibers in the input bundle 42 utilizing
the plurality of arrays of tilting mirrors 46 and 48 to direct the
alignment light beams 58. A loss in optical power of the alignment
light beam 58 in each of the channels is detected via directional
optical couplers 60 and detectors 62. An alignment signal is
generated for each channel based on the loss in optical power of
the alignment light beam 58 detected for that channel. The
alignment signal is utilized to control, via control circuit 64,
the MEMS actuators 52 and 53 associated with each tilting mirror
directing the information light beam 50 for each channel so as to
minimize the optical loss in that channel.
[0029] Both the first and second embodiments of our method are
preferably practiced by dithering the alignment signal to each of
the MEMS actuators and by adjusting the alignment signal based on a
feedback signal that reflects shifts in the gain and phase of the
alignment signal. Also, both methods are preferably practiced by
using a frequency for the alignment signal that is outside a
frequency bandwidth of the information light beam. In addition,
both embodiments are preferably performed in the context of a 3D
MEMS optical switch which requires that four electrical alignment
signals be generated and applied for each channel.
[0030] While we have described two preferred embodiments of our
system and method for actively aligning mirrors in a pure optical
switch, adaptations and modifications thereof will occur to those
skilled in the art. For example, the concept is applicable to any
optical switch wherein beams of light are redirected, by mirrors,
lenses or other movable devices. Both the FIG. 2 and FIG. 3
embodiments could be simplified to provide a 1.times.N optical
switch in which case only a single input fiber would be necessary
along with a single mirror tiltable in two axes in place of the
first array of mirrors. In addition, in the case of the 1.times.N
version of the FIG. 3 embodiment, only a single directional optical
coupler and detector would be needed at the input end of the
system. The system 20 of FIG. 2 could be simplified to use only the
single array of mirrors 26 and associated MEMS actuators 32 so that
the beam of light 30 would be reflected back into a different
optical fiber in the bundle 22 from which it came. The actuators
that tilt the mirrors could be MEMS actuators, piezo-electric
devices, electrostatic devices and hybrids of the same. A
directional coupler is not the most general type of device used to
couple and extract the alignment wavelength. Any optical wavelength
multiplexer/de-multiplexer could be utilized in the FIG. 3
embodiment. For example, a thin film filter (dichroic mirror) could
be used to combine or separate wavelengths. The use of directional
couplers is only representative of our illustrated embodiments. The
use of a thin-film filter would probably be preferable because most
commercially available "band splitter" devices use thin film
technology, rather than directional coupler technology. Therefore,
the protection afforded our invention should only be limited in
accordance with the scope of the following claims.
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